The present invention is for a method for producing a high surface area iron material starting with a low surface area iron metal. The iron material of the present invention has a surface area of at least about 200 m2/g, and is prepared via a method which comprises reacting a low surface area iron metal with oxygen and an organic acid. The high surface area iron material formed via this method is essentially free of contaminants.
Iron-based catalysts are known in the art for use in a variety of chemical reactions. For example, in water gas shift reactions it is common practice to employ chromium-promoted iron catalysts in a high temperature first stage (referred to as a high temperature shift or HTS reaction) to effect carbon monoxide conversion at temperatures above about 350° C. and to reduce the CO content to about 3%-4% (see, for example, D. S. Newsom, Catal. Rev., 21, p. 275 (1980)). A typical composition of high temperature shift (HTS) catalyst comprises from about 60 wt % to about 95 wt % Fe2O3, from about 0 wt % to about 20 wt % Cr2O3, from about 0 wt % to about 10 wt % of CuO and from about 0 wt % to about 10 wt % other active components such as ZrO2, TiO2, CO3O4, Al2O3, SiO2 and/or CeO2.
Since the 1950's iron-based Fischer-Tropsch catalysts have been successfully used in fixed-bed, fluidized-bed and slurry phase reactors, and there have been several methods used for the preparation of iron-based Fischer-Tropsch catalysts. The earliest catalysts, prepared by Fischer, were iron turnings treated with alkali. At high pressure, the liquid product was rich in oxygenated compounds, and at lower pressures hydrocarbons were produced. However, the iron-based catalysts prepared by this method deactivated rapidly.
The most common method of preparation of iron-based Fischer-Tropsch catalysts is precipitation. Typically a solution of an iron salt, such as ferric nitrate, is treated with a base, such as aqueous ammonia or sodium carbonate. The resulting iron oxyhydroxide precipitate is washed and filtered repeatedly to remove salts—ammonium nitrate or sodium nitrate—formed during the precipitation process. The washed filter cake is then dried and calcined. Promotion of the precipitated iron catalyst with copper and a Group I metal can be done at any time, before or after the drying and calcination steps. The final catalyst precursor is usually composed of high surface area corundum phase iron oxide (α-Fe2O3 or hematite).
Other types of iron based catalysts include, fused iron, supported iron and sintered iron. Fused iron catalysts are prepared by melting iron ore and one or more promoter such as SiO2, Al2O3, CaO, MgO and K2O. The resulting catalyst precursor is usually composed predominantly of magnetite (Fe3O4) and has very low surface area. Active fused iron catalysts can only be achieved by reduction of the oxide to metallic iron with hydrogen. The reduced catalyst can have surface area up to about 10 to 15 m2/g. Fused iron catalysts are characterized by high structural integrity and as such are well suited for fluid bed operations; however, the relatively low surface area results in a Fischer-Tropsch catalyst with inferior activity as compared to typical precipitated iron catalysts. Supported iron catalysts are usually prepared by impregnating a solution of an iron salt onto a refractory metal oxide such as Al2O3, SiO2, TiO2 or ZrO2. The impregnation can be carried out by incipient wetness techniques or by excess wetting followed by vacuum drying. Supported iron catalysts can have Fischer-Tropsch activity similar to precipitated iron catalysts on an iron mass basis; however, they are typically inferior on a catalyst volume basis. Supported iron catalysts inevitably suffer from the acidity of the metal oxide supports which increases the selectivity of undesirable methane.
Precipitated iron catalysts are generally regarded as superior Fischer-Tropsch catalysts to the other types of iron catalysts described herein. The major disadvantages of the manufacture of precipitated iron catalysts include high cost, the method is labor intensive, and the by-products are deleterious to the environment. Iron nitrate is the preferred iron source of precipitated iron catalysts because chloride and sulfur contamination from iron chloride or iron sulfate would have a deleterious affect on the activity of the resulting F-T catalyst. Iron nitrate is manufactured by the digestion of iron metal in nitric acid which produces nitrogen oxides that must be recovered by a scrubbing process. This necessary scrubbing step adds additional cost to the process.
A process to produce iron-based Fischer-Tropsch catalysts that reduces or eliminates the washing and filtration steps and has minimal emissions to the environment would be favorable. A logical process from a commercial viewpoint would be to promote, form, dry and calcine a commercially available iron oxide that has high purity and high surface area. Commercial iron oxides are readily available; however, they are usually prepared by treatment of steel with hydrochloric acid or sulfuric acid. These iron oxides contain significant amounts of impurities including chloride and sulfur which makes them unusable as raw materials for Fischer-Tropsch catalysts. As is known in the art, the impurities of the commercial iron oxides (red or yellow iron oxides) can be reduced to a very low level by the pickling process under very high temperatures. However, because of the extreme conditions of the pickling process, the surface area of the iron oxide is generally less than 10 m2/g making the iron oxide unsuitable for some catalyst applications, for example, low temperature Fischer-Tropsch reaction.
Alternatively, a low contaminant iron oxide material may be used, such as the iron oxide taught in U.S. Pat. No. 6,790,274 (issued to Conca et al. on Sep. 14, 2004, and assigned to Süd-Chemie MT), U.S. Patent Application 20040009871 (inventors Hu et al., published on Jan. 15, 2004) and U.S. Patent Application 20040202606 (inventors Conca et al, published on Oct. 14, 2004), all three documents being incorporated in their entirety by reference. However, the iron oxides produced by the process taught in the '274 patent, the '871 application and the '606 application have surface areas of less than about 150 m2/g.